High-purity gases, such as the inert gases, find applications in many and varied sectors of industry. For example, high-purity nitrogen, also referred to as ultrapure nitrogen, is a fluid which is being used more and more in the electronics industry, in the liquid or gas state.
In general, ultrapure nitrogen intended for electronic purposes must be purified, that is to say rid of the impurities or pollutants which may be found in it, for example impurities such as: oxygen (O.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2), hydrogen (H.sub.2), water (H.sub.2 O), hydrocarbons, halogenated compounds, etc.
There are several processes for producing ultrapure nitrogen, the most common of which is that combining oxidative catalysis and cryogenic distillation of atmospheric air. According to this process, the atmospheric air is firstly compressed then heated, generally to a temperature in excess of 120.degree. C., for example by means of heat exchangers such as heaters, before being subjected to oxidative catalysis, so as to oxidize the hydrogen (H.sub.2) and the carbon monoxide (CO) which are found in it into water (H.sub.2 O) and carbon dioxide (CO.sub.2) respectively; the water vapour and the carbon dioxide initially present in the air, and those formed by oxidative catalysis, being subsequently removed by adsorption, for example on an activated alumina bed or a zeolite bed.
It is in fact essential to remove beforehand the impurities such as CO and H.sub.2 present in air, that is to say before cryogenic distillation because, by distillation, these impurities are difficult to separate and entail significant extra costs.
It is furthermore essential to remove the water vapour and the CO.sub.2 found in air, before carrying out cryogenic distillation of the air, in order to eliminate any risk of plugging or clogging the cryogenic distillation columns with these two impurities which are insoluble at cryogenic temperature.
This pretreatment or purification of the air before distillation is referred to as "head purification" because it is upstream of the cryogenic distillation columns.
The air thus purified by catalysis and adsorption is subsequently sent to a cryogenic distillation column, at the outlet of which oxygen, on the one hand, and nitrogen, on the other hand, are recovered.
The nitrogen produced in this way may, if necessary, undergoes other purification steps in order to remove from it all the residual impurities contained therein and thus to meet the specifications required by the electronics industry, namely less than a few ppb (parts per billion by volume) of impurities.
Conventionally, a plant which is capable of implementing a nitrogen production process of this type is successively composed:
of an air compressor making it possible to compress atmospheric air, PA1 optionally one or more heat exchangers intended to heat the air to a temperature compatible with the operation of the catalyst or catalysts, that is to say in general of the order of 80 to 150.degree. C., PA1 a catalytic purification zone comprising one or more catalytic oxidation reactors, containing one or more beds of catalysts operating at high temperature, PA1 one or more heat exchangers intended to cool the air, after passage through the catalytic purification zone, for example to ambient temperature, PA1 a drying/decarbonating zone intended to remove the impurities such as CO.sub.2 and H.sub.2 O from the air before cryogenic distillation, PA1 a cold box comprising, in particular, heat exchangers making it possible to cool the air to a cryogenic temperature, and a cryogenic distillation column intended to separate the air into its constituents nitrogen and oxygen, PA1 and means for recovering, in particular, the nitrogen produced in this way. PA1 (a) compressing the gas flow to a pressure in excess of atmospheric pressure, PA1 (b) bringing the compressed gas flow into contact with at least a first bed of particles of a material containing at least one metal peroxide, and PA1 (c) bringing the gas flow resulting from step (b) into contact with at least a second bed of at least one catalyst, such as an oxidation catalyst and/or a reduction catalyst. PA1 the first bed of particles furthermore contains at least one metal oxide; PA1 the particles in the first bed comprise at least 15% by weight of metal peroxide and, preferably, at least 25% by weight of metal peroxide; PA1 the particles in the first bed comprise metal oxides of at least two metals selected from the transition metals, namely, in particular, Ag, Cu, Mn and Au; PA1 the particles in the first bed consist of at least one mixture comprising a copper oxide, a manganese oxide and a manganese peroxide; PA1 at least step (b) is carried out on an undried gas flow at a temperature greater than or equal to 80.degree. C. and, preferably, greater than or equal to 120.degree. C., or even greater than or equal to 150.degree. C.; PA1 at least step (b) is carried out on a gas flow dried beforehand, at a temperature greater than or equal to 20.degree. C., and preferably greater than or equal to 50.degree. C.; PA1 in step (a), the gas flow is compressed to a pressure of from 3.10.sup.5 Pa to 3.10.sup.6 Pa, preferably to a pressure of from 3.10.sup.5 Pa to 10.sup.6 Pa; PA1 steps (b) and (c) are carried out successively and in the same reactor; PA1 it includes, after step (c), a step of removing oxidized or reduced impurities contained in the gas flow; PA1 it includes, when appropriate, after step (c), a step of cryogenically distilling the gas flow; PA1 it furthermore includes a step of modifying the temperature of the gas flow by heating or by cooling. PA1 the gas flow is air; PA1 the gas flow is an inert gas to be recycled, such as nitrogen, argon, helium or mixtures thereof; PA1 it furthermore includes a step of recovering at least one product selected from nitrogen, oxygen, argon and helium. PA1 means for compressing the gas flow to a pressure in excess of atmospheric pressure, PA1 at least one catalytic zone containing at least one first bed of particles of a material containing at least one metal peroxide, and at least one second bed of at least one catalyst for oxidizing or reducing the impurities, the first bed being placed upstream of the second bed, and PA1 at least one purification zone for removing at least some of the oxidized or reduced impurities.
In general, the catalytic purification zone, operating at high temperature, permits efficient oxidation of the CO and H.sub.2 found in the air into CO.sub.2 and H.sub.2 O, respectively.
However, the intrinsic efficiency of the catalyst employed in this catalytic zone is intimately linked with the nature, that is to say the composition, of the atmospheric air at the site where the cryogenic distillation, and therefore the nitrogen production, are to be carried out.
In fact, although on a standard site the life of a catalyst may be several years, the same is not true on a heavily polluted site, such as a zone with high chemical or oil industry density, for example.
Thus, on certain high-activity industrial sites, very rapid deactivation of the oxidation catalyst has been observable, after only a few months of operation, leading to an almost 50% loss in activity of the catalyst and therefore to its premature replacement.
After an analysis of the atmospheric air, it was observed that it contained high proportions of pollutants which act as "poisons" for the oxidation catalyst and lead to a premature deactivation of it, which is even faster as the amount of the poisons contained in the air is high.
Furthermore, when, after a variable length of purification time, these poisons pass through the catalytic station, they may give rise to premature degradation of the performance of the head purification station located downstream, that is to say, for example, the zeolite bed used for this purpose.
"Poisons" of this type are, in particular, the halogen elements, such as chlorine, fluorine, bromine, etc., the acid gases, such as SO.sub.2 and NO.sub.x, and the oily vapours which may be released by the air compressor when this compressor is of the lubricated type.
The object of the present invention is therefore to improve the processes and devices for purifying gas flows, such as air, by solving the problem of premature deactivation or rapid poisoning of the oxidation or reduction catalysts used for purifying the gas flow.